The method of the invention is specifically designed for use in PMR (Private Mobile Radio or Professional Mobile Radio) networks, i.e. trunked networks, which are typically corporate networks or public authority networks wherein all channels are shared by one or more user organizations. In such networks, the subscribers have individual subscriber numbers and also group numbers indicating the communication group or subscriber group the subscriber belongs to. Hence calls intended for subscribers in a specific group can be switched to members of that group.
The method is suitable for use in mobile communications systems with digital as well as analog radio paths. Analog mobile communications systems are described for example in MPT 1327, A Signalling Standard for Trunked Private Land Mobile Radio Systems, January 1988, revised and reprinted November 1991, and MPT 1343, Performance Specification, January 1988, revised and reprinted September 1991, both published by the United Kingdom Department of Trade and Industry, Radiocommunications Agency.
A group call is one of the essential functions of a private mobile radio system. A group call is used, for example, in all kinds of operations involving more than one participant, particularly when an entire group must be constantly aware of a progression of events. A group call is a conference call in which all participants can speak in turn and hear one another. In group calls, the entire group is called by a common number. A single radio unit, e.g. a mobile unit or a radiotelephone, that is, a subscriber station, may belong to several groups programmed into the radio unit. The programming may be fixed, but it may also be changed by the user of the mobile unit. The system stores information about the base stations related to each group number. The group call may cover one, several or all base stations within the service area of the mobile exchange or several mobile exchanges. When a group call is set up, a traffic channel is allocated at all base stations related to the group, and each of these base stations transmits a group call message comprising the group number and information on the allocated traffic channel. If the mobile unit recognizes the group number included in the group call message, it will switch to the traffic channel indicated in the group call message. Hence, in principle the mobile unit is always available for a group call if it is located within the predetermined operational area of the group and if it is not already engaged in another group call.
In addition to system channels in mobile radio systems, the radio system may employ direct mode channels, i.e. the operation is in direct mode. Subscriber stations communicating in the direct mode do not communicate directly with the radio network or its base stations. Direct mode channels are FDMA frequencies or TDMA time slots on which radiotelephones or other units can communicate outside the system directly with one another, or via repeater stations either with the system base stations or with other mobile units.
Direct mode channels are typically used in situations where, for example, a number of hand portables communicate with one another at a distance from the base station not allowing the system channels to be used.
Another important utility for direct mode channels is in adding capacity when traffic in the system rapidly increases (for example in an incident) in a part of the system service area, for example in a spot-like part thereof.
A direct mode channel is termed a simplex channel or simplex connection. A direct mode channel is a channel that is typically not available for system use at all. It may for example be a channel having a channel spacing equal to that of a mobile communications system, e.g. 12.5 kHz or 25 kHz. Of the radiotelephones communicating on a direct mode channel, the sender has tuned his or her transmitter to the channel and transmits speech or data information. The other radiotelephones that are using direct mode channels have tuned their receivers to the same channel and will hear the transmission directly.
Operation on a direct mode channel may employ analog modulation or digital modulation. A radiotelephone transmitting on a channel may also send signalling information, such as information on user rights and priorities or information about the group communicating on the channel, or data traffic. A direct mode channel may employ encryption or transmit clear speech.
In group call traffic typical to PMR systems, there is a need to provide a normal `free space` feature, since usually two parties are communicating and the others only want to listen. In that case, it would be desirable that these two subscribers who took part in the communication be able to finish their dialogue before a third party can interrupt the communication with his speech item. In prior art approaches, no solution to the above problem has been provided for the operation of mobile units communicating on a direct mode channel.
The problem with the prior art solutions is that there is no system for direct mode channels that would prioritize the start of speech items of mobile units participating in a group call in such a way that for instance two subscribers that just recently participated in communication would be given an opportunity to finish their dialogue before a speaking turn is given to a third subscriber.